correct me if im wrong here someone please, as i have no formal trainign in these matters, just a home amatuer
now power actually comes through those rca's, they only carry sound signals to the amp, to be amplified then carried onto your sub. also adding a sub to your system wont make it any louder per se, just have more bass.
for you to make things louder you need to amplify either the current speakers in which you have now, or add a simple sub and cross-over setup so that your speakers no longer try to play bass they only play mid-bass and higher frequencies.
a simple 2 channel amp and low powered sub can be purchased quite cheaply if your not too concerned about sound quality.
once again, correct me if im wrong someone